As a light control device utilizing a nonlinear optical effect, an optical modulator, an optical switch, etc. have been known. The nonlinear optical effect is a phenomenon showing a nonlinear relation between the electric polarization caused when a strong electric field (optical electric field) is applied to a substance, and the electric field applied. A nonlinear optical material is a material which remarkably shows such nonlinearity.
As a nonlinear optical material utilizing a quadratic nonlinear response, a material which causes second harmonic generation (SHG) or a material which causes Pockels effect (linear electro-optic effect) which produces a change in the refractive index proportionally to the electric field, i.e. an electro-optic effect (EO effect) and the like have been known.
As a nonlinear optical material to be used for the light control device, a nonlinear optical material (a so-called electro-optic polymer) in which an organic compound which develops a nonlinear optical effect is dispersed and oriented by an electric field orientation treatment in a matrix of an organic polymer, has attracted attention in view of easiness of a process for producing a light control device, and easiness of an increase in the area.
The nonlinear optical material is required to have the following properties (i) to (iv).
(i) It is excellent in the nonlinear optical effect.
(ii) The glass transition temperature (Tg) of its matrix is sufficiently high, that is, it is a material excellent in the heat resistance, since if Tg of the matrix is low, orientation of an organic compound to develop a nonlinear optical effect, which is forcibly oriented, is relaxed, and the nonlinear optical effect will be decreased.
(iii) When an organic compound to develop a nonlinear optical effect is oriented in the matrix by electric field orientation treatment, the material is more excellent in the nonlinear optical effect as the voltage applied is higher. If the dielectric voltage of the material is low, short circuiting will occur at the time of the electric field orientation treatment. Thus, the nonlinear optical material is a material having a high dielectric voltage.
(iv) It is excellent in the transparency at a wavelength used (1.3 μm band or 1.5 μm band), in view of small optical transmission loss.
Heretofore, a host-guest nonlinear optical material has been developed, since its material is easily available and it is easily produced. A host-guest nonlinear optical material may be produced by dispersing a compound (guest) having nonlinear optical properties in an optically transparent matrix (host). As the matrix, a polymethacrylate (PMMA) has been actively studied.
However, in a host-guest nonlinear optical material, guest molecules are merely dispersed and there is no bond nor interaction with the matrix, whereby orientation of the guest molecules is thermally unstable and is easily relaxed (Non-Patent Document 1).
Accordingly, it has been studied to suppress relaxation of orientation by bonding the guest molecules to the matrix to limit free movement of the guest molecules. For example, it has been studies to bond the guest molecules to PMMA by a covalent bond (Patent Document 1).
However, the nonlinear optical material the matrix of which is PMMA has a substantial problem such that if it is left to stand at a temperature less than about 100° C. after the electric field orientation treatment, the quadratic nonlinear response will be impaired (FIG. 2 in Non-Patent Document 2, FIG. 4 in Non-Patent Document 3, FIG. 3 in Non-Patent Document 4, FIG. 2 in Non-Patent Document 5). Further, since Tg of PMMA is so low as about 100° C., the process temperature after the electric field orientation treatment is limited to be less than about 100° C. The nonlinear optical material the matrix of which is PMMA has an insufficient dielectric voltage. Further, since PMMA has many C—H bonds, its transparency in 1.3 μm band and 1.5 μm band is insufficient.
As a nonlinear optical material of which the orientation relaxation is suppressed, a nonlinear optical material having a structure to develop a nonlinear optical effect in side chains of a polyimide having high heat resistance has been proposed (Non-Patent Document 6). Further, a nonlinear optical material having crosslinks by Diels-Alder reaction introduced to the matrix has been proposed (Non-Patent Documents 7 and 8).
As a nonlinear optical material which is highly transparent in 1.3 μm band and 1.5 μm band, a material the matrix of which is made of an amorphous fluorinated polymer has been proposed (Patent Document 2).
A nonlinear optical material having a structure to develop a nonlinear optical effect in side chains of a matrix containing fluorine has been proposed (Non-Patent Document 9).